Fix a git issue on case-insensitive file systems

The files sample/HELLOPET.P00 and sample/hellopet.p00 were
binary identical. Their name differ only in upper-/lowercase.
By consequence, this file can only exist once on a case
insensitive file system (which is the default on Mac OS X).

git detects a missing file (it sees only one file) and refuses to sync.

This commit fixes the issue by deleting one file of them
as I do not see a reason for having two binary identical files
with the same name.